Emergency power system, and system for automatically detecting whether or not failure of single cell occurs in battery for use in the same

ABSTRACT

The present invention provides a space-saving and low-cost emergency power system, which can be installed on a narrow place having limited space and employs a battery that is long in service life and low in frequency of replacement. This emergency power system comprises a circuit having a DC load, a rectifier, and a battery, which are electrically connected to one another. This circuit is adapted so that normally, DC power is supplied from the rectifier to the DC load, and the battery is charged, and that in an emergency, such as a power failure, in which an outage of the rectifier occurs, electric power is automatically supplied from the battery to the DC load. In this system, a sodium sulfur battery is employed as the battery.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention generally relates to an emergency powersystem, and to a system for automatically detecting whether not afailure of a single cell occurs in a battery for use in the system.

[0003] 2. Description of the Related Art

[0004] Hitherto, a float charging system employing a lead acid battery30 illustrated in FIG. 10 has been employed as an emergency powersystem. Ordinarily, DC power is supplied from a rectifier 31 to a DCload 32. Moreover, a constant voltage is kept applied to the lead acidbattery 30. Thus, a self-discharged quantity of energy is charged intothe lead acid battery 30. Consequently, the lead acid battery 30 iscontinually kept in a fully charged condition. In the case of emergency,for example, in the event of an outage of the rectifier 31, electricpower is supplied from the DC load 32 to the lead storage battery 30.Thus, in the case of emergency, for instance, in the even of a powerfailure, the conventional emergency power system can supply electricpower to the DC load without momentary outage thereof.

[0005] However, the conventional emergency power system has drawbacks inthat the system requires a large space for placing a lead storagebattery therein, because of low energy density thereof, and that thus,this conventional system cannot be installed on a narrow place havinglimited space. Moreover, the conventional emergency power system hasadditional drawbacks in that the service life of the lead storagebattery is short, that the frequency of replacement of the battery isrelatively high, and that the cost thereof is high.

[0006] The invention is accomplished in view of the aforementioneddrawbacks. Accordingly, an object of the invention is to provide aspace-saving emergency power system that can be placed on a narrow placehaving limited space. Further, another object of the invention is toprovide a low-cost emergency power system employing a battery that islong in service life and low in frequency of replacement. Moreover,another object of the invention is to provide a system for detectingwhether or not a single cell failure occurs in a battery used in anemergency power system.

SUMMARY OF THE INVENTION

[0007] To achieve the foregoing objects, according to an aspect of theinvention, there is provided an emergency power system that comprises acircuit having a DC load, a rectifier, and a battery, which areelectrically connected to one another. The circuit is adapted so thatnormally, DC power is supplied from the rectifier to the DC load, andthe battery is charged. In an emergency, such as a power failure, inwhich an outage of the rectifier occurs, electric power is automaticallysupplied from the battery to the DC load. In this system, the battery isa sodium sulfur battery.

[0008] Further, in an embodiment of the invention, preferably, arectifier output voltage controller for lowering, when the voltage ofthe sodium sulfur battery reaches a level that is equal to or higherthan the predetermined charging voltage, an output voltage of therectifier thereby to automatically discharge energy from the sodiumsulfur battery to the DC load and for raising, when the voltage of thesodium sulfur battery reaches a level that is equal to or lower than thepredetermined discharging voltage, an output voltage of the rectifier tothereby charge the sodium sulfur battery is provided between therectifier and the sodium sulfur battery.

[0009] Further, in an embodiment of the emergency power system of theinvention, preferably, the predetermined charging voltage and thepredetermined discharging voltage of the sodium sulfur battery arewithin a discharge depth range in the case that the composition of anactive material accommodated in a positive electrode chamber of each ofsodium sulfur single cells is in a two-phase region.

[0010] Further, in an embodiment of the invention, it is preferable thatthe sodium sulfur battery is an assembled battery obtained byconstituting strings, each consisting of a predetermined number ofseries-connected single cells, and then parallel-connecting apredetermined number of strings, and that an output voltage of therectifier is controlled by monitoring whether or not a module voltagereaches the predetermined charging voltage and whether or not the modulevoltage reaches the predetermined discharging voltage.

[0011] Furthermore, preferably, an embodiment of the invention detectsan occurrence of a malfunction of a single cell according to whether ornot the difference ΔV (=Vocv−Vm) between the no-load module open circuitvoltage Vocv and the constant-resistance-load module voltage Vm islarger than an initially determined normal voltage difference ΔVn (thatis, ΔV>ΔVn), and generates an alarm when a malfunction of a single celloccurs, thereby to always maintain the function thereof serving as anemergency power system.

[0012] This is because of the facts that the voltage difference in thecase of occurrence of no malfunction ΔV=I·R where I is module current,and R is module resistance. Thus, when a malfunction of a single celloccurs, the resistance R rises, and that thus the voltage difference AVincreases.

[0013] According to the invention, there is provided another emergencypower system, which comprises a circuit consisting of a DC load, arectifier and a battery electrically connected to one another andadapted so that normally, DC power is supplied from the rectifier to theDC load at a constant voltage, and the battery is charged, and that inan emergency including a power failure, in which an outage of therectifier occurs, power is supplied from the battery to the DC load. Inthis system, the battery is a sodium sulfur battery. A controller havinga timer is provided between the rectifier and the sodium sulfur batteryand enabled to interrupt or connect an output of the rectifier so thatwhen the current from the sodium sulfur battery reaches a level that isequal to or lower than a predetermined charging current, an output ofthe rectifier is interrupted, and energy is automatically dischargedfrom the sodium sulfur battery to the DC load for a predeterminedperiod, and that when energy is discharged for the predetermined period,the output of the rectifier is connected to the sodium sulfur battery soas to charge the battery.

[0014] According to the invention, there is provided another emergencypower system, which comprises a circuit consisting of a DC load, arectifier and a battery electrically connected to one another andadapted so that normally, DC power is supplied from the rectifier to theDC load at a constant voltage, and the battery is charged, and that inan emergency including a power failure, in which an outage of therectifier occurs, power is supplied from the battery to the DC load. Inthis system, the battery is a sodium sulfur battery. The power systemfurther comprises a controller enabled to interrupt or connect an outputof the rectifier so that when a voltage of said sodium sulfur batteryreaches a level that is equal to or higher than a predetermined chargingvoltage, an output of the rectifier is interrupted, and energy isautomatically discharged from the sodium sulfur battery to the DC load,and that when the voltage of the sodium sulfur battery reaches a levelthat is equal to or lower than a predetermined discharging voltage, theoutput of the rectifier is connected to the sodium sulfur battery so asto charge the battery.

[0015] According to the invention, there is provided another emergencypower system, which comprises a circuit consisting of a DC load, arectifier and a battery electrically connected to one another, and alsocomprising a diode, the forward direction of electrical conduction ofwhich is a direction from the battery to said DC load, and a firstswitch provided between the rectifier and the sodium sulfur battery.This emergency power is adapted so that normally, DC power is suppliedfrom the rectifier to the DC load, and that in an emergency including apower failure, in which an outage of the rectifier occurs, power issupplied from the sodium sulfur battery to the DC load through thediode. This emergency power system further comprises a control, providedbetween the sodium sulfur battery and the first switch, for putting,after discharge in an emergency, the first switch into a connected stateand for charging the sodium sulfur battery to a predetermined dischargedepth by power supplied from the rectifier, and thereafter interruptingthe first switch to thereby hold said sodium sulfur battery in a statein which power from the rectifier is prevented by the diode, thedirection of electrical conduction of which is a reverse direction, frombeing supplied to the battery.

[0016] Incidentally, in this emergency power system, practical means forputting the first switch into a connected state to thereby charge thesodium sulfur battery to a predetermined discharge depth by supplyingpower from the rectifier is enabled to perform the charging until acharging current reaches a level that is equal to or lower than a setcurrent, or until a module voltage is equal to or lower than apredetermined voltage. It is preferable for reducing a dischargestarting voltage drop in an emergency that upon completion of charging,the discharge depth is increased by discharging a predetermined quantityof electricity (20 to 40 Ah).

[0017] Further, an embodiment of this emergency power system furthercomprises another circuit, provided between said circuit consisting ofthe diode and the first switch electrically parallel-connected and thesodium sulfur battery, for supplying discharging current of the sodiumsulfur battery to a heater, and a second switch, provided between theheater and the sodium sulfur battery. In this system, a temperaturecontrol operation is performed by controlling the on/off of the secondswitch according to the temperature of the sodium sulfur battery underthe control of the controller provided among the sodium sulfur battery,the first switch and the second switch. The sodium sulfur battery ismaintained at a constant discharge depth and at a constant temperatureby repeating one cycle consisting of steps of discharging energy fromthe sodium sulfur battery to the heater, and then charging energy fromthe rectifier to the sodium sulfur battery at each predetermined timeinterval under the control of the controller having a timer, and puttingthe first switch into an interrupted state when the voltage or currentof the sodium sulfur battery reaches a level that is equal to or higherthan a predetermined voltage, or that is equal to or lower than apredetermined current, thereby stopping charging.

[0018] According to the invention, there is provided a detection systemfor automatically detecting an occurrence of a single cell in a batteryused in an emergency power system. The detection system comprises afirst circuit consisting of a DC load, a rectifier, and a sodium sulfurbattery electrically connected to one another, a second circuitconsisting of a diode, the forward direction of electrical conduction ofwhich is a direction from the battery to the DC load, and a first switchparallel-connected to each other, a third circuit, provided between thesecond circuit and the sodium sulfur battery, for supplying adischarging current from the sodium sulfur battery to a heater, and asecond switch provided between the heater and the sodium sulfur battery.The detection system is adapted so that normally, DC power is suppliedfrom the rectifier to the DC load, and that in an emergency including apower failure, in which an outage of the rectifier occurs, power issupplied from the sodium sulfur battery through the diode to the DCload. This detection system further comprises a controller providedamong the sodium sulfur battery, the first switch, and the secondswitch. In this detection system, a temperature control operation isperformed by controlling the on/off of the second switch according to atemperature of the sodium sulfur battery by the controller. The sodiumsulfur battery is maintained at a constant discharge depth and at aconstant temperature by repeating one cycle comprising the steps ofdischarging energy from the sodium sulfur battery to the heater,charging energy from the rectifier to the sodium sulfur battery at eachpredetermined time interval under the control of the controller, whichhas a timer, and putting the first switch into an interrupted state andstopping charging when a voltage or current of the sodium sulfur batteryreaches a level that is equal to or higher than a predetermined voltageor that is equal to or lower than a predetermined current. Thus, anoccurrence of a malfunction of a single cell in the sodium sulfurbattery is automatically detected by monitoring a discharging currentduring energy is discharged from the sodium sulfur battery to theheater.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Other features, objects and advantages of the present inventionwill become apparent from the following description of preferredembodiments with reference to the drawings in which like referencecharacters designate like or corresponding parts throughout severalviews, and in which:

[0020]FIG. 1 is a diagram illustrating an embodiment of an emergencypower system of the invention;

[0021]FIG. 2 is a schematic diagram illustrating the structure of asodium sulfur battery;

[0022]FIG. 3 is a diagram illustrating an example of a sodium sulfurbattery;

[0023]FIG. 4 is a diagram illustrating another embodiment of theemergency power system of the invention;

[0024]FIG. 5 is a diagram illustrating another example of theimprovement of the embodiment of the emergency power system of thepresent invention;

[0025]FIG. 6 is a diagram illustrating an example of charge/dischargecharacteristics of a sodium sulfur single cell of the emergency powersystem of the invention;

[0026]FIG. 7 is a diagram illustrating an example of the relation amongthe charging voltage, the discharging voltage, and the depth of chargeand discharge of a sodium sulfur single cell of the emergency powersystem of the invention;

[0027]FIG. 8 is a diagram illustrating another example ofcharge/discharge characteristics of a sodium sulfur single cell of theemergency power system of the invention;

[0028]FIG. 9 is a diagram illustrating an example of the relation amongthe charging voltage, the discharging voltage, and the depth of chargeand discharge of a sodium sulfur single cell of the emergency powersystem of the invention; and

[0029]FIG. 10 is a diagram illustrating a conventional emergency powersystem.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] Hereinafter, the preferred embodiments of the invention will bedescribed in detail by referring to the accompanying drawings. However,needless to say, the invention is not limited to the followingembodiments.

[0031] An emergency power system of the invention features theemployment of a sodium sulfur battery as a power storage battery.

[0032] As illustrated in FIG. 1, the emergency power system of theinvention has a circuit constituted by electrically connecting arectifier 2, a sodium sulfur battery 1, and a DC load 3 to one another.Normally, DC power is supplied from the rectifier 2 to the DC load 3.Moreover, the sodium sulfur battery 1 is charged. When a power failureoccurs, electric power is supplied from the sodium sulfur battery 1 tothe DA load 3.

[0033] At ordinary times, the charging of the sodium sulfur battery 1 bythe rectifier 2 is performed until the voltage level of the sodiumsulfur battery 1 reaches a level that is equal to or higher than apredetermined level. A controller 4 is provided in the circuit andoperative to control the voltage level of the rectifier 2 according tothe voltage supplied from the sodium sulfur battery 1. The voltage levelof the rectifier 2 is lowered at a moment when the voltage level of thesodium sulfur battery 1 reaches a predetermined charging voltage. Then,energy is automatically discharged from the sodium sulfur battery 1 tothe DC load 3 until the voltage level of the battery 1 reaches a levelthat is equal to or lower than a predetermined discharging voltage. At amoment at which the battery 1 reaches the predetermined dischargingvoltage, the voltage level of the rectifier 2 is raised. Thus, thesodium sulfur battery 1 is charged up to the predetermined chargingvoltage.

[0034] The system of the invention features that an output voltage ofthe rectifier 2 is controlled so that the sodium sulfur battery 1 isrepeatedly and alternatively charged and discharged between thepredetermined voltages, in other words, within a constant dischargedepth range in the sodium sulfur single cell. TABLE 1 Lead acid BatteryNAS battery Item MSE Type G50 kW Chemical Substance Pb Na NegativeElectrode PbO₂ S Positive H₂SO₄ β-alumina Electrode ElectrolyticSolution (Electrolyte) Cell Voltage 2.0 V 2.075 V Nominal Operating−15˜45° C. 280˜360° C. Temperature Energy Density Wh/kg 32.6 110 Wh/l 66130 Self-Discharge ≦0.1%/day (25° C.) None Rate Expected Life 7˜9 years15 years Maintenance Maintenance-Free Maintenance-Free Water Supply,Specific Gravity measurement, Even Charge, Component ReplacementEfficiency (Initial) Ah Efficiency 90% 100% Wh Efficiency 75% 70%Internal 0.10 Ω · Ah 0.7 Ω · Ah Resistance Float Charging Permitted(When Protection Circuit Not Performed, Is Needed Capacity Is (CheckingIs Reduced Due to Needed) Accidental Discharge) Merits Immediate HighEnergy Utilization Density Possible No Self- Room Temperature DischargeOperation Possible High Energy Float Charging Efficiency PossibleDefects Low Energy Temperature Density Rising Needed High Self- HighTemperature Discharge Rate Operation Short Life Heat ReservingGeneration of Power Is Needed Hydrogen by during Unused Overcharge Floatcharging Low Energy Impossible Efficiency High Internal Resistance

[0035] As is seen from TABLE 1 for making the comparison between thesodium sulfur battery and the lead storage battery, the energy densityof the sodium sulfur battery is extremely high and nearly four timesthat of the lead storage battery. Therefore, the system of the inventionemploys a sodium sulfur battery as the storage battery. Consequently,the system of the invention can exceedingly save space.

[0036] However, the float charging cannot be applied to the sodiumsulfur battery, differently from the lead storage battery. In the caseof the lead storage battery, a constant voltage is applied from therectifier thereto. Thus, even when the lead storage battery iscontinuously charged in a fully charged state, water contained inelectrolytic solution is dissolved. Thus, the charging energy isabsorbed thereto, so that the lead storage battery does not malfunction.

[0037] In contrast, in the sodium sulfur battery, as illustrated in FIG.2, a single cell structure has a positive electrode chamber 8 and anegative electrode chamber 7, which are partitioned off by a bottomedsolid electrolyte tube 6 made of β-alumina. A positive electrodeconductive material 9 impregnated with sulfur serving as a positiveelectrode active material is accommodated in the positive electrodechamber 8, while sodium serving as a negative electrode active materialis accommodated in the negative electrode chamber 7. The single cell ismade to operate at 300° C. or so. Further, when discharged, sodium ischanged into sodium ions that move in the β-alumina tube 6 and reactwith sulfur accommodated in the positive electrode to thereby generatesodium polysulfide Na₂S_(x). Incidentally, the composition of the sodiumpolysulfide varies with a discharge depth.

[0038] On the other hand, in the case of charging, the sodium ionscontained in the positive electrode chamber 8 move in the β-alumina tube6 and are then returned to the negative electrode chamber 7 as sodiummetal. In the case that the charging is still continued in a state inwhich sodium ions in the positive electrode chamber 8 are extremelyreduced, a high applied voltage is concentrated on the β-alumina tube 6.This causes a trouble that the β-alumina tube 6 is damaged, or that thesodium ions of a β-alumina crystal move to the negative electrode andthus cannot be maintained. In the case of discharging, similar troublesoccur when the discharge is excessively continued.

[0039] Thus, according to the invention, the charge and discharge of thesodium sulfur battery are repeated within a discharge depth range inwhich the battery does not malfunction. It is preferable for preventingan occurrence of a malfunction of the battery that the predeterminedcharging voltage and the predetermined discharging voltage are those inthe case where the composition of the active material accommodated inthe positive electrode chamber is in a two-phase region.

[0040] Further, as described in TABLE 1, the sodium sulfur batteryfeatures that the energy efficiency thereof is relatively high, ascompared with the lead storage battery. It is preferable for loweringpolarization resistance and enhancing the energy efficiency still moreto set the predetermined charging voltage and the predetermineddischarging voltage at values in the case that the composition of theactive material accommodated in the positive electrode chamber is in thetwo-phase region.

[0041] Moreover, as described in TABLE 1, the sodium sulfur battery hasa long service life, which is twice that of the lead storage battery ormore. Thus, the employment of the sodium sulfur battery can reduce thecost of the emergency power system.

[0042] Further, it is preferable for preventing an occurrence of amalfunction of a single cell that the sodium sulfur battery is anassembled battery obtained by constituting strings, each of which isobtained by series-connecting a predetermined number of sodium sulfursingle cells, and then parallel-connecting a predetermined number of thestrings, as illustrated in FIG. 3, and that the system has a controllerfor controlling the charge and discharge of the sodium sulfur battery byperforming the rise and fall of the voltage of the rectifier when themodule voltage reaches a predetermined voltage.

[0043] Furthermore, it is preferable for always maintaining thefunctions of the emergency power system that an occurrence of amalfunction of a single cell is detected by monitoring whether or notthe difference ΔV (=Vocv−Vm) between a no-load module open circuitvoltage Vocv, a constant-resistance-load module voltage Vm and a normalvoltage difference ΔVn meet the condition ΔV>ΔVn, and that an alarm isgenerated when a malfunction of a single cell occurs.

[0044] Further, another embodiment of the invention may have acontroller for interrupting or connecting an output of the rectifier,instead of the controller for controlling the rise and fall of thevoltage of the rectifier when the voltage of the sodium sulfur batteryreaches the predetermined charging voltage or the predetermineddischarging voltage.

[0045] Electric power is automatically supplied in such an embodimentfrom the sodium sulfur battery to the DC load by interrupting the outputof the rectifier when the voltage of the sodium sulfur battery reaches alevel that is equal to or higher than the predetermined chargingvoltage. In the case that the output of the rectifier is connected tothe battery when the voltage of the sodium sulfur battery becomes lowerthan the predetermined discharging voltage or lower, the sodium sulfurbattery is charged by the rectifier again. Thus, a space-savingemergency power system is realized without a malfunction of the sodiumsulfur battery by having a controller for monitoring the charging anddischarging voltages of the sodium sulfur battery and for interruptingor connecting the output of the rectifier.

[0046]FIG. 4 is a diagram illustrating the configuration of anotherembodiment of the invention. As shown in FIG. 4, this embodiment has acircuit in which a DC load 3, a general-purpose rectifier 14, and asodium sulfur battery 1 are electrically connected to one another.Moreover, a circuit consists of a diode 11, the forward direction ofelectrical conduction of which is a direction from the sodium sulfurbattery 1 to the DC load, and a first switch, which areparallel-connected, and is provided between the general purposerectifier 14 and the sodium sulfur battery 1. At ordinary times, DCpower is supplied from the general purpose rectifier 14 to the DC load3. In an emergency, such as an outage of the general purpose rectifier14 at a power failure, electric power is supplied from the sodium sulfurbattery 1 to the DC load 3 through the diode 11.

[0047] After the discharge of the sodium sulfur battery 1 in theemergency, the first switch 12 is put into a connected state by thecontroller 13 provided between the sodium sulfur battery 1 and thisswitch 12, so that the sodium sulfur battery 1 is charged to a level,which is equal to or higher than the predetermined charging voltage orcurrent, by supplying electric power from the general purpose rectifier14 thereto. The emergency power system 15 is adapted so that when thevoltage level of the battery 1 reaches a level that is equal to or lowerthan the predetermined charging voltage or current, the first switch 12is brought into an interrupted state, and that the sodium sulfur battery1 is maintained in a state in which is no voltage is applied from thegeneral purpose rectifier 14 thereto.

[0048] The emergency power system 15 of this type does not alwaysperform the charge/discharge cycle of the sodium sulfur battery 1. Thissystem automatically discharges the sodium sulfur battery 1 to thepredetermined discharging voltage, which is the limit, only in anemergency. After the discharge, the system maintains the battery 1 insuch a state. At normal times, the system puts the first switch 12 intoa connected state to thereby charge the battery 1 to the predeterminedvoltage. The charging operation is manually or automatically performedafter an occurrence of a power failure in an emergency.

[0049] The emergency power system of this type has an advantage in thatthe general purpose rectifier used in the conventional emergency powersystem can be utilized in this embodiment without no modification.

[0050]FIG. 5 is a diagram illustrating another embodiment that is theimprovement of the embodiment of the invention illustrated in FIG. 4.The embodiment of the invention illustrated in FIG. 5 has a circuit forsupplying discharging current to a heater 17, which is provided betweenthe circuit consisting of the diode 11 and the first switch 12, whichare parallel-connected, and the sodium sulfur battery 1. This embodimentalso has a second switch 16 provided between the heater 17 and thesodium sulfur battery 1, and a controller 18, provided between thesodium sulfur battery 1 and each of the first switch 12 and the secondswitch 16, for controlling the interruption and connection of theswitches 12 and 16.

[0051] In such a circuit, the on/off of the second switch 16 iscontrolled according to the temperature of the sodium sulfur battery 1.Then, the temperature is maintained by discharging energy from thesodium sulfur battery 1 to the heater 17. The controller 18 having atimer causes the general purpose rectifier 14 at each predetermined timeinterval to charge the sodium sulfur battery 1. When the voltage of thecharged battery reaches a level that is equal to or higher than thepredetermined voltage or that is equal to or lower than thepredetermined current, the first switch 12 is put into an interruptedstate. Then, the battery 1 is charged. This process is employed as onecycle. The emergency power system 19 features that the sodium sulfurbattery 1 is maintained at a constant discharge depth and at a constanttemperature by repeating this cycle.

[0052] The emergency power system of this type has advantages in thatwhether or a malfunction of a single cell occurs can be always monitoredand the functions thereof including the self-diagnostic function can bealways maintained because electric current is fed to and flows throughthe heater, in addition to the advantage that the rectifier used in theconventional emergency power system using the lead storage battery canbe used therein without modification.

[0053] Furthermore, the emergency power system of this type has anotheradvantage in that the heater can be used as a heat reserving heater forthe sodium sulfur battery.

[0054] Hereinafter, the embodiments of the invention are described.

[0055] First Embodiment

[0056]FIGS. 6 and 7 illustrate the characteristics and operations ofthis embodiment that uses the circuit shown in FIG. 1 and is adapted torepeat the charge/discharge of the sodium sulfur battery by controllingthe output voltage of the rectifier and also repeat the charge/dischargeof a sodium sulfur single cell in a range between the constant dischargedepths.

[0057] In the case of the single cell operated at an operatingtemperature of 320° C., the predetermined charging voltage was 2.17 V.Further, the predetermined discharging voltage was 1.98 V. The dischargedepth was 100 Ah to 350 Ah. The discharging current was 73 A. Thecharging current was 64 A. Furthermore, the total energy capacity andthe charge capacity of each of the cells of the sodium sulfur batteryare 62.4 kWh and 600 Ah/52 cells, respectively.

[0058] Second Embodiment

[0059]FIGS. 8 and 9 illustrate the characteristics and operations ofthis embodiment that uses the circuit shown in FIG. 5 and is adapted tocontrol the on/off of the second switch according to the temperature ofthe sodium sulfur battery and to maintain the temperature by dischargingenergy from the sodium sulfur battery to the heater, and to uses thecontroller having a timer, which causes the general purpose rectifier ateach predetermined time interval to charge the sodium sulfur battery,and to establish a process as one cycle consisting of steps of puttingthe first switch into an interrupted state when the voltage of thecharged battery reaches a level that is equal to or higher than thepredetermined voltage or that is equal to or lower than thepredetermined current, and of then charging the battery and to alsorepeat this cycle thereby maintaining the sodium sulfur battery 1 at aconstant discharge depth and at a constant temperature and also repeatthe charge/discharge of the sodium sulfur single cell in a range betweenthe constant discharge depths.

[0060] In the case of the single cell operated at an operatingtemperature of 320° C., the predetermined charging voltage was 2.17 V.Further, the predetermined discharging voltage was 1.98 V. The dischargedepth was 100 Ah to 350 Ah. The discharging current was 73 A. Thecharging current was 64 A. Furthermore, the total energy capacity andthe charge capacity of each of the cells of the sodium sulfur batteryare 62.4 kWh and 600 Ah/52 cells, respectively.

[0061] As described above, the emergency power system of the inventioncan save space and achieve long service life thereof and reduce themaintenance cost thereof. Thus, a low-cost emergency power system isrealized. Further, the system for automatically detecting an occurrenceof a single cell failure in the battery used in the emergency powersystem of the invention has a self-diagnostic function ofself-diagnosing a storage battery fault. Thus, the emergency powersystem of the invention has an advantage in that the function of servingas an emergency power supply can be maintained at all times.

[0062] Although the preferred embodiments of the present invention havebeen described above, it should be understood that the present inventionis not limited thereto and that other modifications will be apparent tothose skilled in the art without departing from the sprint of theinvention.

[0063] The scope of the present invention, therefore, should bedetermined solely by the appended claims.

What is claimed is:
 1. An emergency power system comprising a circuithaving a DC load, a rectifier, and a battery, which are electricallyconnected to one another, wherein said circuit is adapted so thatnormally, DC power is supplied from said rectifier to said DC load, andsaid battery is charged, and that in an emergency including a powerfailure, in which an outage of said rectifier occurs, electric power isautomatically supplied from said battery to said DC load, wherein saidbattery is a sodium sulfur battery.
 2. An emergency power systemaccording to claim 1 , which further comprises a rectifier outputvoltage controller for lowering, when a voltage of said sodium sulfurbattery reaches a level that is equal to or higher than a predeterminedcharging voltage, an output voltage of said rectifier thereby toautomatically discharge energy from said sodium sulfur battery to saidDC load and for raising, when the voltage of said sodium sulfur batteryreaches a level that is equal to or lower than a predetermineddischarging voltage, an output voltage of said rectifier to therebycharge said sodium sulfur battery, which controller is provided betweensaid rectifier and said sodium sulfur battery.
 3. An emergency powersystem according to claim 2 , wherein the predetermined charging voltageand the predetermined discharging voltage of said sodium sulfur batteryare within a discharge depth range in a case that composition of anactive material accommodated in a positive electrode chamber of each ofsodium sulfur single cells is in a two-phase region.
 4. An emergencypower system according to claim 2 , wherein said sodium sulfur batteryis an assembled battery obtained by constituting strings, eachconsisting of a predetermined number of series-connected single cells,and then parallel-connecting a predetermined number of strings, andwherein an output voltage of the rectifier is controlled by monitoringwhether or not a module voltage reaches the predetermined chargingvoltage and whether or not the module voltage reaches the predetermineddischarging voltage.
 5. An emergency power system according to claim 4 ,wherein an occurrence of a malfunction of a single cell is detectedaccording to whether or not a difference between a no-load module opencircuit voltage and a constant- resistance-load module voltage is largerthan an initially determined normal voltage difference, and an alarm isgenerated at an occurrence of a malfunction of a single cell, therebyalways maintaining a function of serving as an emergency power system.6. An emergency power system comprising a circuit consisting of a DCload, a rectifier and a battery electrically connected to one anotherand adapted so that normally, DC power is supplied from said rectifierto said DC load at a constant voltage, and said battery is charged, andthat in an emergency including a power failure, in which an outage ofsaid rectifier occurs, power is supplied from said battery to said DCload, wherein said battery is a sodium sulfur battery, and wherein acontroller having a timer is provided between said rectifier and saidsodium sulfur battery and enabled to interrupt or connect an output ofsaid rectifier so that when electric current from said sodium sulfurbattery reaches a level that is equal to or lower than a predeterminedcharging current, an output of said rectifier is interrupted, and energyis automatically discharged from said sodium sulfur battery to said DCload for a predetermined period, and that when energy is discharged forsaid predetermined period, the output of said rectifier is connected tosaid sodium sulfur battery so as to charge said battery.
 7. An emergencypower system comprising a circuit consisting of a DC load, a rectifierand a battery electrically connected to one another and adapted so thatnormally, DC power is supplied from said rectifier to said DC load at aconstant voltage, and said battery is charged, and that in an emergencyincluding a power failure, in which an outage of said rectifier occurs,power is supplied from said battery-to said DC load, wherein saidbattery is a sodium sulfur battery, said power system further comprisinga controller enabled to interrupt or connect an output of said rectifierso that when a voltage of said sodium sulfur battery reaches a levelthat is equal to or higher than a predetermined charging voltage, anoutput of said rectifier is interrupted, and energy is automaticallydischarged from said sodium sulfur battery to said DC load, and thatwhen the voltage of said sodium sulfur battery reaches a level that isequal to or lower than a predetermined discharging voltage, the outputof said rectifier is connected to said sodium sulfur battery so as tocharge said battery.
 8. An emergency power system comprising a circuitconsisting of a DC load, a rectifier and a battery electricallyconnected to one another, and also comprising a diode, the forwarddirection of electrical conduction of which is a direction from saidbattery to said DC load, and a first switch provided between saidrectifier and said sodium sulfur battery, said emergency power adaptedso that normally, DC power is supplied from said rectifier to said DCload, and that in an emergency including a power failure, in which anoutage of said rectifier occurs, power is supplied from said sodiumsulfur battery to said DC load through said diode, said emergency powersystem further comprising: a control, provided between said sodiumsulfur battery and said first switch, for putting, after discharge in anemergency, said first switch into a connected state and for chargingsaid sodium sulfur battery to a predetermined discharge depth by powersupplied from said rectifier, and thereafter interrupting said firstswitch to thereby hold said sodium sulfur battery in a state in whichpower from said rectifier is prevented by said diode, the direction ofelectrical conduction of which is a reverse direction, from beingsupplied to said battery.
 9. An emergency power system according toclaim 8 , which further comprises: another circuit, provided betweensaid circuit consisting of said diode and said first switch electricallyparallel-connected and said sodium sulfur battery, for supplyingdischarging current of said sodium sulfur battery to a heater; and asecond switch, provided between said heater and said sodium sulfurbattery, wherein a temperature control operation is performed bycontrolling on/off of said second switch according to a temperature ofsaid sodium sulfur battery under the control of said controller providedamong said sodium sulfur battery, said first switch and said secondswitch, wherein said sodium sulfur battery is maintained at a constantdischarge depth and at a constant temperature by repeating one cycleconsisting of steps of discharging energy from said sodium sulfurbattery to said heater, and then charging energy from said rectifier tosaid sodium sulfur battery at each predetermined time interval under thecontrol of said controller having a timer, and putting said first switchinto an interrupted state when a voltage or current of said sodiumsulfur battery reaches a level that is equal to or higher than apredetermined voltage, or that is equal to or lower than a predeterminedcurrent, thereby stopping charging.
 10. A detection system forautomatically detecting an occurrence of a single cell in a battery usedin an emergency power system, said detection system comprising: a firstcircuit consisting of a DC load, a rectifier, and a sodium sulfurbattery electrically connected to one another; a second circuitconsisting of a diode, the forward direction of electrical conduction ofwhich is a direction from said battery to said DC load, and a firstswitch parallel-connected to each other; a third circuit, providedbetween said second circuit and said sodium sulfur battery, forsupplying a discharging current from said sodium sulfur battery to aheater; and a second switch provided between said heater and said sodiumsulfur battery, said detection system being adapted so that normally, DCpower is supplied from said rectifier to said DC load, and that in anemergency including a power failure, in which an outage of saidrectifier occurs, power is supplied from said sodium sulfur batterythrough said diode to said DC load, said detection system furthercomprising: a controller provided among said sodium sulfur battery, saidfirst switch, and said second switch, wherein a temperature controloperation is performed by controlling on and off operations of saidsecond switch according to a temperature of said sodium sulfur batteryby said controller, wherein said sodium sulfur battery is maintained ata constant discharge depth and at a constant temperature by repeatingone cycle comprising the steps of discharging energy from said sodiumsulfur battery to said heater, charging energy from said rectifier tosaid sodium sulfur battery at each predetermined time interval under thecontrol of said controller, which has a timer, and putting said firstswitch into an interrupted state and stopping charging when a voltage orcurrent of said sodium sulfur battery reaches a level that is equal toor higher than a predetermined voltage or that is equal to or lower thana predetermined current, and wherein an occurrence of a malfunction of asingle cell in said sodium sulfur battery is automatically detected bymonitoring a discharging current during energy is discharged from saidsodium sulfur battery to said heater.